1. Field of the Invention
The present invention relates to humidification arrangements used to control the humidity of breathable gases used in respiratory apparatus ventilation systems including invasive and non-invasive ventilation, Continuous Positive Airway Pressure (CPAP), Bi-level therapy and treatment for sleep disordered breathing (SDB) conditions such as Obstructive Sleep Apnea (OSA), and for various other respiratory disorders and diseases.
2. Description of the Art
Respiratory apparatus commonly have devices to alter the humidity of the breathable gas in order to reduce drying of the patient's airway and consequent patient discomfort and associated complications. The use of a humidifier placed between the positive airway pressure device (or flow generator) and the patient mask produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort.
Many humidifier types have been proposed, including humidifiers that are either integrated with or configured to be coupled to the relevant respiratory apparatus. While passive humidifiers can provide some relief, generally a heated humidifier is required to provide sufficient humidity and temperature to the air so that patient will be comfortable.
Humidifiers typically comprise a water tub having a capacity of several hundred milliliters, a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the positive airway pressure device, and a gas outlet adapted to be connected to a gas conduit that delivers the humidified pressurized gas to the patient's mask.
Tub-of-water humidifiers are vulnerable to liquid water spillage if they are not maintained in a substantially vertical orientation. Spillage of liquid water can either travel into the gas conduit to the patient or back into the positive airway pressure device and associated electronics or deplete the reservoir of humidifying water. In either of the cases, the spillage of water is undesirable.
A semi-permeable membrane may be used to isolate the liquid water from the gas flow. The semi-permeable membrane has the characteristic of allowing water vapour to pass through it but not liquid water. Water vapour passing through the semi-permeable membrane may be entrained into the gas flow within the humidifier and then passed to the patient.
U.S. Pat. Nos. 3,871,373, 4,146,597 and 4,155,961 disclose the use of tubes of semi-permeable membrane, containing liquid water, inserted into the gas flow path. The tubes are used as a source of water vapour for entraining into the passing gas flow stream.
U.S. Pat. Nos. 4,753,758 and 4,921,642 disclose the use of a semi-permeable membrane to separate a water chamber and a gas flow path chamber of a humidifier. In these examples, the water and gas flow chambers are typically generous in their volumes with consequently a high thermal capacity and slow thermal response.
U.S. Pat. Nos. 4,910,384 and 5,062,145 disclose a heater situated within a water-containing envelope of semi-permeable membrane material, dividing the gas chamber in two.
In U.S. Pat. No. 4,657,713, a heater block of the humidifier incorporates a water supply and a semi-permeable filter membrane.
None of these prior art devices provide a satisfactory solution to the provision of humidified breathable gas to the patient, nor to the ease of construction, disposability, retrofitting and hygiene requirements for a humidification device.